• dark adaptation, photopic visual fields, frequency doubling perimetry, ETDRS visual acuity, development or increase of neovascularization, need for panretinal photocoagulation, development of vitreous or preretinal hemorrhage [ Time Frame: 2 years ] [ Designated as safety issue: No ]
  

• area of retinal thickening, central subfield thickness, macular volume, central retinal artery equivalent (CRAE), central retinal vein equivalent (CRVE), arteriovenous ratio (AVR = CRAE/CRVE) [ Time Frame: 2 years ] [ Designated as safety issue: No ]
  

The objectives of this proof-of-concept study are to investigate whether doxycycline can 1) slow the deterioration or improve retinal function and/or 2) induce regression, or slow progression, of diabetic retinopathy. The tests will be performed in the Ophthalmology Departments of the Penn State College of Medicine and Glostrup Hospital, Copenhagen, Denmark. The 24 month proof-of-concept clinical study will involve a prospective, randomized, double-masked clinical trial including 60 adult patients with type 1 or type 2 diabetes who have severe non-proliferative diabetic retinopathy (ETDRS level 53E) or mild or moderate proliferative diabetic retinopathy (retinal and /or optic disk neovascularization less than the "high-risk" ETDRS level 61 or 65), neovascularization of the disc or neovascularization elsewhere >1/2 disc area and in whom panretinal photocoagulation is not imminently required in the ophthalmologist's judgment.

Systemic Exclusion Criteria:

• unstable medical status (e.g. glycemic control, blood pressure, cardiovascular disease) in the opinion of investigator
• significant renal disease (defined as a serum creatinine > 2.5 mg/dL),
• systolic blood pressure > 180 mm Hg or diastolic blood pressure > 110 mm Hg
• history of headaches associated with tetracycline therapy
• history of pseudotumor cerebri
• pregnancy; for women of child-bearing potential, a serum pregnancy test will be performed.
• lactating or intending to become pregnant during the study period (at least 24 months)
• sexually active women of child-bearing potential not actively practicing birth control by using a medically accepted device or therapy (that is, intrauterine device, hormonal contraceptive, or barrier devices) during the study period (at least 24 months); since doxycycline may interfere with the effectiveness of hormonal contraceptives, sexually active women of child-bearing potential who use a hormonal contraceptive will be required to use a second form of contraception to safeguard against contraceptive failure while participating in the study
• known allergy/intolerance to doxycycline or any ingredient in the study drug or placebo (e.g. cellulose, hypromellose, iron oxide, methacrylic acid copolymer, polyethylene glycol, polysorbate 80, sugar spheres, talc, titanium dioxide, and triethyl citrate)
• patients taking phenytoin, barbiturates or carbamazepine, with gastroparesis, with a history of gastrectomy, gastric bypass surgery or otherwise deemed achlorhydric or with a BMI > 30 kg/m2 will also be excluded because of altered doxycycline pharmacokinetics and/or bioavailability
• patients taking strontium, acitretin or tretinoin will be excluded due to the potential for serious drug interactions with doxycycline
• patients with abnormal ALT or AST at baseline will be referred to their primary care physician for medical clearance for participation in this study